Computer driven plotters are in common use for various types of engineering and graphics display. Typically, a pen dispenses ink on a medium such as paper, velum, etc., and both the pen and paper move during the process. The pen itself will move transverse to the direction of paper motion, and the paper motion is typically in both directions along its length. When the pen moves alone, horizontal lines (i.e., transverse to the longitudinal axis of the paper) will be displayed. When just the paper moves, the lines on the paper will be vertical. When both the pen and the paper move simultaneously, various curves can be drawn based on the relative motion in these two directions. Obviously, it is essential that the mechanism which grasps the paper does so securely and without slippage.
Most paper transport mechanism used with plotters grasp the page or medium to be drawn upon at longitudinally extending outboard edges. A lower surface of the paper's edge is contacted by a driving shaft which is coated with an abrasive compound which actually secures the paper along the area of contact, impressing a unique tractive pattern on the paper corresponding to the abrasive pattern on the shaft. In order to effectively distort the paper in this manner, a pressure roller overlies the drive shaft at that point of paper penetration and presses the paper against the drive shaft. As the paper advances back and forth in the plane of travel, the impression created in the paper by the abrasive driving shaft must always contact the driving shaft at precisely the same points to assure alignment with the corresponding transverse motion of the pen for quality plotting. In effect, the driving shaft imparts a friction "gear" pattern on the paper with which it always meshes when it drives the paper along the paper's longitudinal axis.
The universally accepted industry practice is shown in FIG. 1. The medium M is interposed between a drive shaft DS and a pressure roller PR. As the drive shaft DS moves in the direction of the double-ended arrow A, the medium M moves in accordance with the arrow C. In order to reliably drive the medium M, a substantial force F must be present between the pressure roller PR and the drive shaft DS. Typically, this force is in the order of 9 to 20 pounds of pressure applied by the pressure roller PR. Two observations are in order. First, in order to impart this pressure F, an angular force B must be delivered to the pressure roller through an interconnected pressure roller arm. The pressure roller arm is carried on a support shaft PSS which also carries and supports the pen. Note that there is a reaction force F' imposed upon the shaft PSS.
Second, the force F imposed by the pressure roller PR must be resisted by the driving shaft DS in some manner. As this is a close tolerance system, it is important to not have the drive shaft flex. Thus, the prior art provides a plurality of shaft support rollers SSR strategically placed under the drive shaft DS. However, since the pressure roller arm is capable of being placed along the longitudinal axis of the pen support shaft, this solution is not entirely satisfactory.
Thus, larger diameter driving shafts have been used to resist shaft flexing. Typically, the design of the mechanism which turns the driving shaft must take into account the shaft's weight. This results from the necessity in having the shaft change direction frequently and accelerate quickly. Thus, hollow tubes with relatively thick walls are used to reduce weight. Although larger tubes having greater diameters can in most cases resolve the problems associated with shaft flexing, they do so at the expense of response time for the shaft. With the increased diameter, inertia and angular momentum inefficiencies increase.